Bottom Line:
During reading, we generate saccadic eye movements to move words into the center of the visual field for word processing.Our approach is based on iterative computation of the proportions of several types of oculomotor errors, the underlying probabilities for word-targeting, and corrected distributions of landing positions.These results show that fixation probabilities are strongly affected by oculomotor errors.

Affiliation: Department of Psychology, University of Potsdam, Potsdam, Germany. Ralf.Engbert@uni-potsdam.de

ABSTRACTDuring reading, we generate saccadic eye movements to move words into the center of the visual field for word processing. However, due to systematic and random errors in the oculomotor system, distributions of within-word landing positions are rather broad and show overlapping tails, which suggests that a fraction of fixations is mislocated and falls on words to the left or right of the selected target word. Here we propose a new procedure for the self-consistent estimation of the likelihood of mislocated fixations in normal reading. Our approach is based on iterative computation of the proportions of several types of oculomotor errors, the underlying probabilities for word-targeting, and corrected distributions of landing positions. We found that the average fraction of mislocated fixations ranges from about 10% to more than 30% depending on word length. These results show that fixation probabilities are strongly affected by oculomotor errors.

Mentions:
The simulations started in step 0, where we used the experimentally observed skipping rates (Fig. 3a, black line) as the intended skipping rates (light blue). However, the numerical simulations showed that mislocated fixations strongly bias skipping probability as a function of word length in two ways. First, the realized saccade can undershoot the intended word (failed skipping) and, second, the saccade can overshoot the intended word (unintended skipping). In the simulations, failed skippings (Fig. 3a, red line) turned out to be more frequent than unintended skippings (green line). Therefore, over the full range of word lengths, realized skipping probabilities (dark blue line) are smaller than the values observed experimentally. To tackle this problem, our algorithm adjusted word-targeting probabilities in the next iteration, i.e., the algorithm increased the intended skipping probability for the next run of the oculomotor model.

Mentions:
The simulations started in step 0, where we used the experimentally observed skipping rates (Fig. 3a, black line) as the intended skipping rates (light blue). However, the numerical simulations showed that mislocated fixations strongly bias skipping probability as a function of word length in two ways. First, the realized saccade can undershoot the intended word (failed skipping) and, second, the saccade can overshoot the intended word (unintended skipping). In the simulations, failed skippings (Fig. 3a, red line) turned out to be more frequent than unintended skippings (green line). Therefore, over the full range of word lengths, realized skipping probabilities (dark blue line) are smaller than the values observed experimentally. To tackle this problem, our algorithm adjusted word-targeting probabilities in the next iteration, i.e., the algorithm increased the intended skipping probability for the next run of the oculomotor model.

Bottom Line:
During reading, we generate saccadic eye movements to move words into the center of the visual field for word processing.Our approach is based on iterative computation of the proportions of several types of oculomotor errors, the underlying probabilities for word-targeting, and corrected distributions of landing positions.These results show that fixation probabilities are strongly affected by oculomotor errors.

Affiliation:
Department of Psychology, University of Potsdam, Potsdam, Germany. Ralf.Engbert@uni-potsdam.de

ABSTRACTDuring reading, we generate saccadic eye movements to move words into the center of the visual field for word processing. However, due to systematic and random errors in the oculomotor system, distributions of within-word landing positions are rather broad and show overlapping tails, which suggests that a fraction of fixations is mislocated and falls on words to the left or right of the selected target word. Here we propose a new procedure for the self-consistent estimation of the likelihood of mislocated fixations in normal reading. Our approach is based on iterative computation of the proportions of several types of oculomotor errors, the underlying probabilities for word-targeting, and corrected distributions of landing positions. We found that the average fraction of mislocated fixations ranges from about 10% to more than 30% depending on word length. These results show that fixation probabilities are strongly affected by oculomotor errors.